The present invention relates to a polycrystalline thin film having a crystalline structure of a type C rare earth oxide with a well-aligned crystal orientation and a method of producing the same, and an oxide superconductor element of excellent superconducting properties having a polycrystalline thin film which has a crystalline structure of a type C rare earth oxide with a well-aligned crystal orientation and an oxide superconducting layer and a method of producing the same.
Oxide superconducting materials which have been discovered in recent years are good superconducting materials which have critical temperatures above the liquid nitrogen temperature. However, there remain various problems to be solved before oxide superconducting materials can be used as practical superconductors. One of the problems is the low critical current densities of oxide superconducting materials.
The problem that the critical current density of the oxide superconducting material is low stems mainly from the electrical anisotropy which is intrinsic to the crystals of the oxide superconducting material. It is known that the electric conductivity of oxide superconducting materials is high in the a-axis and b-axis directions of the crystal but is low in the c-axis direction. Thus, in order to use an oxide superconducting layer formed on a substrate as a superconductor element, it is necessary to form an oxide superconducting layer with a good crystal orientation on a substrate and to align the a-axis or b-axis of the crystal of the oxide superconducting material with the intended direction of current flow, while aligning the c-axis of the oxide superconducting material with the other direction.
Accordingly, such a practice has been employed wherein an intermediate layer having a good crystal orientation and made of MgO, SrTiO3 or the like is formed on a substrate such as a metal tape by means of a sputtering apparatus, and an oxide superconducting layer is formed on the intermediate layer. However, the oxide superconducting layer formed on this type of intermediate layer by a sputtering apparatus has a critical current density (typically about 1,000 to 10,000 A/cm2) which is far lower than that of the oxide superconducting layer (typically several hundred thousands of A/cm2) which is formed on a single crystal substrate made of such a material. The cause of this problem is assumed to be as follows.
FIG. 14 is a sectional view of an oxide superconductor element made by forming an intermediate layer 2 on a substrate 1 made of a polycrystalline material in the form of a metal tape or the like by means of a sputtering apparatus, and then by forming an oxide superconducting layer 3 on the intermediate layer 2 by the sputtering apparatus. In the structure shown in FIG. 14, the oxide superconducting layer 3 is in a polycrystalline state in which a multitude of crystal grains 4 are bonded together in a random manner. These crystal grains 4 individually show the c-axis of each crystal being oriented perpendicular to the substrate surface, but the a-axis and b-axis are randomly oriented.
When the a-axes and b-axes are randomly oriented among the crystal grains of the oxide superconducting layer, degradation in the superconducting properties, particularly in the critical current density, would be caused since quantum coupling of the superconducting state is lost in the grain boundaries in which the crystal orientation is disturbed.
The cause of the oxide superconductor element turning into a polycrystalline state with the a-axes and b-axes randomly oriented is assumed to be as follows: since the intermediate layer 2 formed below the oxide superconductor element is polycrystalline in which the a-axes and b-axes are randomly oriented, the oxide superconducting layer 3 would be grown in such a condition so as to match the crystal structure of the intermediate layer 2.
The present inventors have found that an oxide superconductor element having a sufficient critical current density can be produced by forming an intermediate layer of YSZ (yttrium-stabilized zirconia), which has a well-oriented a-axis and b-axis, on a polycrystalline substrate by means of a special process, and by forming an oxide superconducting layer on the intermediate layer. With respect to this technology, the present inventors have filed applications by way of Japanese Unexamined Patent Application, First Publication No. Hei 4-293464, Japanese Patent Application, First Publication No. Hei 8-214806, Japanese Unexamined Patent Application, First Publication No. Hei 8-272606, and Japanese Unexamined Patent Application, First Publication No. Hei 8-272607.
The technology proposed in these patent applications makes it possible, when a film is formed on a polycrystalline substrate using a target made of YSZ, to selectively remove YSZ crystals of an unfavorable crystal orientation by means of an ion beam-assisted process in which the film forming surface of the polycrystalline substrate is irradiated in an oblique direction with a beam of ions, such as Ar+, thereby selectively depositing YSZ crystals of a good crystal orientation, so that an intermediate layer of YSZ crystals having a good crystal orientation is formed.
According to the technology proposed in the previous applications of the present inventors, a polycrystalline thin film of YSZ with the a-axes and b-axes being favorably oriented can be made. It was also verified that the oxide superconducting material formed on the polycrystalline thin film has a sufficient critical current density, and the present inventors began research into developing technology of producing better polycrystalline thin films from other materials.
FIG. 15 is a sectional view showing an example of the oxide superconductor element which the inventors have been using recently. An oxide superconductor element D of this example has a four-layer structure made by forming, with the technology described previously, an orientation control intermediate layer 6 of YSZ or MgO on a substrate 5 in the form of a metal tape, then forming a reaction stopper intermediate layer 7 made of Y2O3 thereon, and forming an oxide superconducting layer 8 thereon.
The reason for using the four-layer structure is that, in order to make an oxide superconducting layer having a composition of Y1Ba2Cu3O7xe2x88x92x, it is necessary to apply a heat treatment at a temperature of several hundred degrees centigrade after forming the oxide superconducting layer having the desired composition by sputtering or another film forming process, but diffusion of the elements may proceed between the oxide superconducting layers having the compositions of YSZ and Y1Ba2Cu3O7xe2x88x92x, due to the heat supplied during the heat treatment; the diffusion may cause deterioration of the superconducting properties and must be prevented. The YSZ crystals which constitute the orientation control intermediate layer 6 have a cubic crystal structure, and the oxide superconducting layer having a composition of Y1Ba2Cu3O7xe2x88x92x has a crystal structure called perovskite. Both of these crystal structures belong to a class of face-centered cubic crystals and have similar crystal lattices, but there exists a difference of about 5% in the lattice size between the two structures. For example, the distance between the nearest atoms, namely the distance between an atom located at a corner of the cubic lattice and an atom located at the center of the face of the cubic lattice, is 3.63 xc3x85 (0.363 nm) for YSZ, 3.75 xc3x85 (0.375 nm) for Y2O3, and 3.81 xc3x85 (0.381 nm) for an oxide superconducting layer having the composition of Y1Ba2Cu3O7xe2x88x92x. Thus, Y2O3 has an intermediate value between those of YSZ and Y1Ba2Cu3O7xe2x88x92x and is useful for bridging the difference in lattice size and can be advantageously used as a reaction stopper layer due to the similarity of the compositions.
With the four-layer structure shown in FIG. 15, however, the number of required layers increases which leads to a problem of increasing the number of production processes.
In order to form a reaction stopper intermediate layer 7 of favorably oriented Y2O3 crystals directly on the metal tape substrate 5, the present inventors tried to form the reaction stopper intermediate layer 7 of Y2O3 on the substrate 5 by applying ion beam-assisted technology for which they had previously filed a patent application. However, the reaction stopper intermediate layer 7 of favorably oriented Y2O3 crystals could not be formed under the film growing conditions of conventional ion beam-assisted technology.
Meanwhile, techniques to form various films of a good orientation on polycrystalline substrates have been used in fields other than the application of oxide superconducting materials, such as thin optical films, magneto-optical disks, circuit wiring boards, high-frequency waveguides, high-frequency filters and cavity resonators. In any of these fields, it remains a challenge to form a favorably oriented polycrystalline thin film of stable film quality on a substrate. A polycrystalline thin film having a satisfactory crystal orientation would make it possible to improve the quality of optical thin films, magnetic films or thin films for circuit wiring to be formed thereon. It will be more preferable to be capable of forming thin optical films, thin magnetic films or thin films for circuit wiring, which have a satisfactory crystal orientation, directly on the substrate.
The present invention has been made to solve the problems described above, and has been completed after intensively studying the methods to form a polycrystalline layer of a type C rare earth oxide, such as Y2O3, having a favorable crystal orientation on a substrate by applying ion beam-assisted technology for which the present inventors had previously filed a patent application. An object of the present invention is to provide a polycrystalline thin film comprising crystal grains of a type C rare earth oxide having a favorable crystal orientation which makes it possible to align the c-axes of the oxide crystal grains of the type C rare earth oxide with a direction perpendicular to the substrate surface whereon the thin film is to be formed, and to align the a-axes and b-axes of the crystal grains of the type C rare earth oxide with a plane parallel to the film forming surface of the substrate. Another object of the present invention is to provide an oxide superconductor element which has a polycrystalline thin film comprising crystal grains of a type C rare earth oxide having a favorable crystal orientation, and an oxide superconducting layer having a favorable crystal orientation.
In order to achieve the objects described above, the polycrystalline thin film of the present invention consists mainly of oxide crystal grains which have a crystal structure of a type C rare earth oxide represented by one of the formulas Y2O3, Sc2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, Lu2O3, and Pm2O3, formed on the film forming surface of a polycrystalline substrate wherein the grain boundary inclination angles (the grain boundary misalignment angle) between the same crystal axes of different crystal grains in the polycrystalline thin film along the plane parallel to the film forming surface of the polycrystalline substrate are controlled within 30 degrees.
In the constitution described above, the polycrystalline substrate can be formed from a heat resistant metal tape made of an Ni alloy, and the polycrystalline thin film can be formed from Y2O3.
In order to achieve the objects described above, the present invention provides a method of producing a polycrystalline thin film comprising oxide crystal grains which have a crystal structure of a type C rare earth oxide represented by one of the formulas Y2O3, Sc2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, Lu2O3, and Pm2O3, being formed on the polycrystalline substrate surface whereon the thin film is to be formed, with the grain boundary inclination angles(the grain boundary misalignment angle) between the same crystal axes of different crystal grains along the plane parallel to the surface of the polycrystalline substrate whereon the film is to be formed being controlled within 30 degrees, wherein the polycrystalline substrate is set to a temperature in a range from 200 to 400xc2x0 C. and an ion beam of Kr+ or Xe+ ions or a combination of these ions is generated from an ion source with the energy of the ion beam being set in a range from 100 eV to 300 eV, while the incident angle of the ion beam irradiating the substrate is set in a range from 50 to 60 degrees when depositing the particles generated from the target, which is made of the same elements as those of the polycrystalline thin film, onto the polycrystalline substrate.
In order to achieve the objects described above, the oxide superconductor element of the present invention comprises a polycrystalline substrate, a polycrystalline thin film formed on the polycrystalline substrate surface, and an oxide superconducting layer formed on the polycrystalline thin film, with the polycrystalline thin film consisting of oxide crystal grains which have a crystal structure of a type C rare earth oxide represented by one of the formulas Y2O3, Sc2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, Lu2O3, and Pm2O3, wherein the grain boundary inclination angles between the same crystal axes of different crystal grains along the plane parallel to the film forming surface of the polycrystalline substrate are controlled within 30 degrees.
In order to achieve the objects described above, the present invention provides a method of producing an oxide superconductor element which comprises a polycrystalline substrate, a polycrystalline thin film formed on the polycrystalline substrate surface, and an oxide superconducting layer formed on the polycrystalline thin film, with the polycrystalline thin film consisting of oxide crystal grains which have a crystal structure of a type C rare earth oxide represented by one of the formulas Y2O3, Sc2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, Lu2O3, and Pm2O3, with the grain boundary inclination angles between the same crystal axes of different crystal grains along the plane parallel to the surface of the polycrystalline substrate whereon the film is to be formed being controlled within 30 degrees, wherein the polycrystalline substrate is set to a temperature in a range from 200 to 400xc2x0 C. and an ion beam of Kr+ or Xe+ ions or a combination of these ions is generated from an ion source with the energy of the ion beam being set in a range from 100 eV to 300 eV, while the incident angle of the ion beam irradiating the film forming surface of the substrate is set in a range from 50 to 60 degrees when depositing the particles generated from the target, which is made of the same elements as those of the polycrystalline thin film, onto the polycrystalline substrate, and then the oxide superconducting layer is formed on the polycrystalline thin film.
The polycrystalline thin film of a type C rare earth oxide, such as Y2O3, formed on the polycrystalline substrate is considered to be more advantageous than the conventional polycrystalline thin film of YSZ in many respects when a superconducting layer made of an oxide is formed thereon.
First, the lattice constant of ZrO2 which is the main component of the YSZ crystal is 5.14 xc3x85 (0.514 nm) and assuming that the distance between an atom located at the center of a face of the unit cell and an atom located at a corner of the unit cell (the distance between nearest atoms) in the face-centered cubic lattice of ZrO2 is 3.63 xc3x85 (0.363 nm), then the lattice constant of the Y2O3 crystal is 5.3 xc3x85 (0.53 nm) and the distance between the nearest atoms is 3.75 xc3x85 (0.375 nm). Taking into account the fact that the distance between the nearest atoms of an oxide superconducting material having the composition of Y1Ba2Cu3O7xe2x88x92x is 3.9 xc3x85 (0.39 nm) and that the lattice constant is from 5.4 to 5.5 xc3x85 (0.54 to 0.55 nm) which is 2xc2xd (the square root of 2) times the size of 3.9 xc3x85 (0.39 nm), the polycrystalline thin film of Y2O3 is considered to be more advantageous with respect to crystal matching than the polycrystalline thin film of YSZ. That is, when depositing the atoms of the polycrystalline thin film by the ion beam-assisted process, normal depositing of the atoms would be more easily achieved by using a material having a smaller value of the distance between the nearest atoms. Also, because Y2O3 has the crystal structure of a type C rare earth oxide, a material having a crystal structure of a type C rare earth oxide represented by one of the formulas Sc2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, Lu2O3, and Pm2O3, can be used.
The research conducted by the present inventors shows that BaZrO3 is likely to be generated by thermal diffusion, due to heating during the production process or a heat treatment, in the interface between the polycrystalline thin film of YSZ and the oxide superconducting layer of Y1Ba2Cu3O7xe2x88x92x, while the interface between the polycrystalline thin film of Y2O3 and the oxide superconducting layer of Y1Ba2Cu3O7xe2x88x92x is stable under conditions of heating to a temperature from about 700 to 800xc2x0 C., and therefore, a polycrystalline thin film of Y2O3 is also promising in this regard.
According to the present invention, the polycrystalline thin film comprising crystal grains of a type C rare earth oxide such as Y2O3, which has a favorable crystal orientation and is formed on the polycrystalline substrate with the grain boundary inclination angles(the grain boundary misalignment angle) being controlled within 30 degrees, can be preferably used as a base for forming various thin films thereon, and makes it possible to achieve good superconductive properties for the case when the thin film to be formed is a superconductive layer, can achieve good optical properties for the case when the thin film to be formed is an optical film, can achieve good magnetic properties for the case when the thin film to be formed is a magnetic film, and can obtain a thin film of lower wiring resistance and less defects for the case when the thin film to be formed is used for circuit wiring.
The oxide of a type C rare earth oxide used in the polycrystalline thin film may be an oxide represented by one of the formulas Y2O3, Sc2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, Lu2O3, and Pm2O3.
A heat resistant metal tape made of an Ni alloy may be used as the polycrystalline substrate, and a metal tape carrying thereon the polycrystalline thin film comprising crystal grains of a type C rare earth oxide, such as Y2O3, can be made.
According to the present invention, since the substrate is controlled to a temperature in a range from 200 to 400xc2x0 C., the energy of the ion beam is set in a range from 100 eV to 300 eV, and the incident angle of the ion beam irradiating the substrate is set in a range from 50 to 60 degrees from the normal direction of the film forming surface when depositing the particles generated from the Y2O3 target onto the polycrystalline substrate, it becomes possible to form the polycrystalline thin film of Y2O3 directly on the polycrystalline substrate with a good crystal orientation, which has been impossible in the prior art.
Since the polycrystalline thin film of Y2O3 can be formed directly on the polycrystalline substrate, it is not necessary to further laminate with a YSZ polycrystalline thin film, and the number of laminations required for forming a satisfactory film of a good crystal orientation on a polycrystalline substrate is reduced, thereby contributing to the simplification of the production process.
When the oxide superconducting layer is formed on the polycrystalline thin film of Y2O3, which has a good crystal orientation as described above, an oxide superconducting layer having a good crystal orientation can be formed, and therefore, an oxide superconducting layer having a high critical current density and a high critical current can be made. This is because the polycrystalline thin film of Y2O3 has better crystal matching characteristics with the oxide superconducting layer than the polycrystalline thin film of YSZ, thus making it possible to make an oxide superconducting layer having a better crystal orientation than in the case of using a polycrystalline thin film of YSZ.
Moreover, according to the present invention, since the polycrystalline thin film of Y2O3 can be formed directly on the polycrystalline substrate, the number of laminates constituting the oxide superconductor element can be reduced thereby simplifying the production process, in comparison with the prior art in which a double-layered film of YSZ and Y2O3 is used in consideration of the heat treatment which is carried out after forming the oxide superconducting layer.
Since it has been found from research conducted by the present inventors that BaZrO3 is likely to be generated in the interface between the polycrystalline thin film of YSZ and the oxide superconducting layer of Y1Ba2Cu3O7xe2x88x92x by thermal diffusion due to heat treatment or the like yet the interface between the polycrystalline thin film of Y2O3 and the oxide superconducting layer of Y1Ba2Cu3O7xe2x88x92x is stable under the conditions of heating to a temperature from about 700 to 800xc2x0 C., the polycrystalline thin film of Y2O3 is also advantageous in this regard and makes it possible to provide an oxide superconductor element which is less prone to the degradation of the superconducting properties even when subjected to a heat treatment after forming the oxide superconducting layer.